2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridine carboxylate; imazapyr (ISO)

Long-term toxicity to fish

Description of key information

In an early-life stage test a NOAEC of 43.1 mg/L (based on mean measured concentrations) in rainbow trout (Oncorhynchus mykiss) was observed. In another early-life stage test with fathead minnow (Pimephales promelas) a NOEC of 118 mg/L (based mean measured concentrations) was observed. And a full life-cycle test with fathead minnows revealed a NOEC of 120 mg/L (based on mean measured concentrations).

Additional information

Key study (American Cyanamid Company, 971-87-155, 1989)

An early-life stage test was conducted equivalent to OECD guideline 210 to estimate the chronic effects of the test substance on the rainbow trout (Oncorhynchus mykiss). Embryos were exposed to nominal test substance concentrations of 0, 6.25, 12.5, 25.0, 50.0 and 100 mg/L, which corresponds to the mean measured concentrations of <5.0, 6.59, 12.1, 24.0, 43.1 and 92.4 mg/L. The criterion of effect in the early life stage test were hatching success of embryos and growth and survival of juveniles for 28 days post swim-up. Under the condition of this early life-stage study in rainbow trout, a concentration of 92.4 mg/L resulted in reduced hatch and reduced fry survival (LOAEC). No effects were noted at a concentration of 43.1 mg/L (NOAEC). The study did not meet all guideline requirements (feeding limited the growth of replicates with higher fish densities).

 

Supporting study (American Cyanamid Company, ECO 97-102, 1998)

An early-life stage flow-through test was conducted to estimate the chronic effects of the test substance on the fathead minnow (Pimephales promelas). Fathead minnows were exposed to the test substance at nominal concentrations of 0, 7.5, 15, 30, 60 and 120 mg/L (corresponding to measured concentrations: >0.1, 7.4, 15, 31, 62 and 118 mg/L). Viability and hatchability of the eggs, and survival and growth of the larvae were evaluated as toxic endpoints. The test substance produced no treatment-related effects on embryonic survival, time to hatch, alevin survival, terminal length, or wet and dry weight. The NOEC was determined to be 118 mg/L (mean measured; nominal: 120 mg/L).

Supporting study (American Cyanamid Company, 954-97-101, 1999)
Additionally, in a full life-cycle flow-through test the effects of the test substance in fathead minnows (Pimephales promelas) were evaluated. Fathead minnows were exposed to nominal test substance concentrations of 0, 7.5, 15, 30, 60 and 120 mg/L. The test concentrations remained extremely stable throughout the test. Mean measured concentrations in the parental exposure were <LOD, 7.2, 15, 29, 59 and 120 mg/L; whereas mean measured concentrations during the second generation exposure were <LOD, 7.5, 15, 30, 61 and 120 mg/L. The endpoints addressed for the parental generation were: hatching success, survival, growth and reproduction. The endpoints evaluated for the second generation were hatching success, survival and growth. The test substance produced no treatment-related effects on growth, embryo survival, time to hatch or larval and juvenile survival of the F0 and F1 generations. No treatment-related effects were observed on percent spawning frequency, mean number of eggs produced per female or mean percent fertilization success. Consequently the NOEC was 120 mg/L, the highest concentration tested.

 

Conclusion:

Although, the early life stage test with rainbow trout did not meet all guideline requirements, it is used as key study due to worst case considerations.